This invention relates to semiconductor technology and in particular to the formation of a high-frequency capacitor on a semiconductor substrate.
Higher frequencies are increasingly being used in communications technology. For example, frequencies in the range of 450 MHz to 3 GHz are used in cellular communications and frequencies in the range of 10 GHz to 18 GHz are used in satellite video and data transmission.
These applications require small, precise capacitors. Multi-layer ceramic capacitors have been employed for this purpose, but they tend to be lacking in precision and performance. Thin film capacitors have improved precision and performance but they are expensive.
Accordingly, there is a need for a precision high-frequency capacitor that can be manufactured at a reasonable cost.
In accordance with this invention, a precision high-frequency capacitor is formed on a heavily-doped semiconductor substrate having first and second principal surfaces. The capacitor includes a dielectric layer on the first principal surface of the substrate and a main electrode layer on the dielectric layer. A conductive layer is formed on the second principal surface of the substrate. A via containing a conductive material extends through the substrate. A second electrode layer is formed over the first principal surface of the substrate, adjacent an opening of the via. The second electrode is electrically connected to the conductive layer by means of the conductive material in the via. Thus, when a voltage difference is applied to the electrodes, the main electrode layer and the substrate act as the xe2x80x9cplatesxe2x80x9d of the capacitor, separated by the dielectric layer.
In an alternative embodiment, the via is omitted, and the second electrode layer, electrically insulated from the first electrode layer, is formed over the first principal surface of the substrate. In one version, the second electrode is separated from the substrate by the dielectric layer, creating in effect a pair of series-connected capacitors, with the substrate representing the common terminal between the capacitors. In another version, the second electrode is in electrical contact with the substrate, creating a single capacitor. Each of the electrode layers may include a plurality of fingers, with the fingers being interdigitated. The dielectric layer, often an oxide, may be thinner under the fingers than under the xe2x80x9cpalmxe2x80x9d portions of the electrode layers from which the fingers protrude.
Capacitors in accordance with this invention exhibit numerous advantages as compared with prior art capacitors. They can be fabricated at a wafer level with a very low effective series resistance (ESR). They can function at very tight tolerances (e.g.,  less than 2%) throughout their operational range and can operate at very high frequencies (e.g., up to 5 GHz and higher). They can have a quality (Q) factor, for example, that is much higher than 1000 at 1 MHz.